Incoherently beam combined (IBC) lasers combine the output from an array of gain elements or emitters (typically consisting of semiconductor material, such as GaAlAs, GaAs, InGaAs, InGaAsP, AlGaInAs, and/or the like, which is capable of lasing at particular wavelengths) into a single output beam that may be coupled into, for example, an optical fiber. The gain elements may be discrete devices or may be included on an integrated device. Due to the geometry of IBC lasers, each gain element tends to lase at a unique wavelength.
FIG. 1 depicts a prior art arrangement of components in IBC laser 10. IBC laser includes emitters 12-1 through 12-N associated with fully reflective surface 11. Emitters 12-1 through 12-N are disposed in a substantially linear configuration that is perpendicular to the optical axis of collimator 15 (e.g., a lens). Collimator 15 causes the plurality of beams produced by emitters 12-1 through 12-N to be substantially collimated and spatially overlapped on a single spot on diffraction grating 16. Additionally, collimator 15 directs feedback from partially reflective 17 via diffraction grating 16 to emitters 12-1 through 12-N.
Diffraction grating 16 is disposed from collimator 15 at a distance approximately equal to the focal length of collimator 15. Furthermore, diffraction grating 16 is oriented to cause the output beams from emitters 12-1 through 12-N to be diffracted on the first order toward partially reflective component 17, thereby multiplexing the output beams. Partially reflective component 17 causes a portion of optical energy to be reflected. The reflected optical energy is redirected by diffraction grating 16 and collimator 15 to the respective emitters 12-1 through 12-N. Diffraction grating 16 angularly separates the reflected optical beams causing the same wavelengths generated by each emitter 12-1 through 12-N to return to each respective emitter 12-1 through 12-N. Accordingly, diffraction grating 16 is operable to demultiplex the reflected beams from reflective component 17.
It shall be appreciated that the geometry of external cavity 13 of IBC laser 10 defines the resonant wavelengths of emitters 12-1 through 12-N. The center wavelength (xcexi) of the wavelengths fed back to the ith emitter 12-i is given by the following equation: xcexi=A[sin(xcex1i)+sin(xcex2)]. In this equation, A is the spacing between rulings on diffraction grating 16, xcex1i is the angle of incidence of the light from the ith emitter on diffraction grating 16, and xcex2 is the output angle which is common to all emitters 12-1 through 12-N. As examples, similar types of laser configurations are also discussed in U.S. Pat. No. 6,208,679.
To allow emitters 12-1 through 12-N to operate in this type of configuration, anti-reflective coating 14 is applied to the front facet of emitters 12-1 through 12-N. Anti-reflective coating 14 allows substantially all incident light to be transmitted. By applying anti-reflective coating 14, emitters 12-1 through 12-N lase at the wavelength defined by the feedback wavelengths as discussed above. Specifically, it shall be appreciated that emitters 12-1 through 12-N do not operate as Fabry-Perot emitters, since anti-reflective coating 14 does not provide a partially reflective surface to create internal feedback.
Moreover, anti-reflective coatings of appreciable quality (possessing a reflectivity on the order of 10xe2x88x924) are difficult to achieve on a consistent basis. This is problematic, since anti-reflective coatings of lower quality can significantly diminish performance of an IBC laser.
Additionally, the use of anti-reflective components increases the difficulty of verifying the performance of components in an IBC laser. Specifically, it is desirable to verify the performance of each emitter prior to assembling the entire laser. Performance verification of an emitter array is performed by applying current through the emitters of the emitter array and measuring the output optical power over a period of time. If a very low reflectivity is applied to the front facet, the emitter array will not generate a significant amount of optical power and performance verification is not possible. As a result, the entire IBC laser must be assembled before the various components can be tested. Accordingly, this greatly increases the cost of manufacturing IBC lasers.
The present invention is directed to a system and method which utilize an incoherently beam combined (IBC) laser. The IBC laser includes a plurality of emitters with each of the emitters possessing a partially reflective surface on their front facet. The partially reflective surface causes resonant wavelengths to be defined. In certain embodiments, the system and method arrange the external cavity and emitter spacings of the IBC laser such that the center feedback wavelength provided to each emitter is an etalon resonant wavelength. In other embodiments, the range of feedback wavelengths is adapted so that it exceeds the free spectral range (the separation in wavelength space between adjacent etalon resonant wavelengths).